Hook flash or flash is a button on a telephone

Telephone

The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...

 that simulates quickly hanging up then picking up again (a quick off-hook

Off-hook

In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...

/on-hook

On-hook

In telephony, the term on-hook has the following meanings:# The condition that exists when a telephone or other user instrument is not in use, i.e., when idle waiting for a call. Note: on-hook originally referred to the storage of an idle telephone receiver, i.e., separate earpiece, on a switchhook...

/off-hook cycle). This action can signal the telephone exchange

Telephone exchange

In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...

 to do something. A common use of hook flash is to switch to another incoming call with the call waiting

Call waiting

Call waiting , in telephony, is a feature on some telephone networks. If a calling party places a call to a called party which is otherwise engaged, and the called party has the call waiting feature enabled, the called party is able to suspend the current telephone call and switch to the new...

 service. Another use is to indicate a request for voice conferencing. For example, a user may use a procedure like the following to initiate three-way calling:

1. Pick up phone handset (causing the line to be off-hook
   Off-hook
   In telephony, the term off-hook has the following meanings:# The condition that exists when a telephone or other user instrument is in use, i.e., during dialing or communicating. Note: off-hook originally referred to the condition that prevailed when telephones had a separate earpiece , which hung...
   
   ).
2. Hear a dialtone
3. Dial the first number and greet the first party
4. Press the hook flash button (or quickly tap the on-hook sensor on the phone)
5. Hear a stutter dial tone (a series of beeps followed by another dialtone)
6. Dial the second number and greet the second party
7. Press the hook flash button again.

The second "flash" signals the Central Office Switch to link the two active conversations, so that all three parties are connected to the same logical telephone line.

In contrast to PBX conferencing systems, the two calls are joined at the Central Office switch, rather than at the customer premises PBX.

History

The switchhook is the device that senses whether the handset or receiver is in its cradle. The term "flash" originated from the cord circuit

Cord circuit

In telecommunication, a cord circuit is a switchboard circuit in which a plug-terminated cord is used to establish connections manually between user lines or between trunks and user lines. A number of cord circuits are furnished as part of the switchboard position equipment. The cords may be...

 of the early telephone switchboard

Telephone switchboard

A switchboard was a device used to connect a group of telephones manually to one another or to an outside connection, within and between telephone exchanges or private branch exchanges . The user was typically known as an operator...

 that telephone company operators used to connect calls. The calling party

Calling party

The calling party is a person who initiates a telephone call over the public switched telephone network, usually by dialing a telephone number....

 and called party

Called party

The called party is a person who answers a telephone call. The person who initiates a telephone call is the calling party....

 each had an indicator light on the cord circuit

Local loop

In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network...

. When the subscriber cycled the telephone on-hook/off-hook, the light would flash. Actors in old movies often demonstrate the method, seeking the operator's attention. The user does a tap-tap-tap. When an operator comes on the line, the actor says, "Hello? Operator? We've been cut off." Then the operator attempts to reestablish the connection. The flashing light of this equipment of bygone days is the origin of the phrase "flashing the switchhook".

Centrex

Centrex

Centrex is a portmanteau of central exchange, a kind of telephone exchange.In the United Kingdom, British Telecom markets this service as FeatureLine .-Use of Centrex:...

telephones added a hook-flash button in the 1960s after some users incorrectly attempted the attendant-recall function.